The invention relates to heating or cooking apparatus, and, more particularly, to an improved tunnel oven for heating food or other products while being conveyed from a tunnel entrance through the oven to the tunnel exit within a preselected time.
It is previously been known to provide multiple tier ovens of the tunnel type. Such an oven is disclosed in co-assigned U.S. Pat. No. 4,008,996 of Harold D. Wells, entitled "Multiple Tier Oven". Said patent discloses a multiple tier oven with a plurality of superimposed tunnel-form oven chambers. Each of the tunnels is provided with a plurality of infrared emitter panels along the roof thereof for heating purposes which are of the form disclosed in U.S. Pat. No. 3,809,859 of Harold D. Wells. The panels radiate heat downwardly into their respective chambers, but also cause heating the floor, or hearth, of the next higher chamber, the lowermost chamber having both roof and floor heating panels. A single thermocouple is associated with the panels of each tier for sensing the temperature thereof, such thermocouple being connected with a temperature controller which controls the electrical energization of resistance elements of each of the panels. In that prior art arrangement, all of the heating panels of a tier thus are controlled by a single temperature sensing means.
Accordingly, in this prior art oven arrangement a single temperature control is associated with each tier to provide a means for setting a preselected temperature. The control provides time-proportioning energization of the resistance elements of the panels in accordance with the setting by switching the electrical power on and off at a relatively rapid interval in accordance with the temperature sensed by the sensing means associated with each tier.
While the oven described in said U.S. Pat. No. 4,008,996 has proven to be extraordinarily advantageous in numerous and attained great commercial success, it has been found that certain limitations are necessarily imposed upon its flexibility by design features inherent therein. As alluded to above, the infrared emitter panels which constitute the roof of each tier provide some heating of the floor (or hearth) of next higher tier, but such heating is at the expense of accurate temperature control of the higher tier if the difference between the temperatures selected for each tier exceeds a given value. Thus, flexibility in use is limited to some extent by the need to operate the oven so that the temperature set for each tier does not exceed a certain differential between adjacent tiers.
It has been found desirable to employ in ovens of the presently disclosed character a tunnel of the form having a metal hearth comprised of thermally conductive sheet material, such as steel. Moreover, it is now found to be desirable to utilize the hearth as one source of heat for cooking a food article through the tunnel while utilizing the infrared emitter panels of the roof as another source of heat for cooking of the food articles. Under these circumstances, it has been found frequently advantageous to operate the oven with the hearth of a given tier at a different preselected temperature from the temperature of the roof infrared emitter panels. Such feature could not be conveniently provided with a multiple tier oven of the type noted in the above identified patent without limiting the differences in temperatures between different tiers.
While it is desirable to provide such flexibility in the operation of an oven as described above, it is a matter of importance to ensure that temperatures within the oven chamber, i.e., the tunnel of each tier, be maintained relatively constant. Otherwise, cooking of food articles as desired in accordance with the preset temperature will not be effected, with the further possibility that food products cooked at different times may be cooked to different degrees of doneness so as to result in nonuniformity of heated food products.
When cold food articles are introduced to the tunnel entrance, it is found that heat transfer between the heat sources, i.e., roof heater panels and the hearth, and the cold food products is considerable. This condition is aggravated when the food products are frozen. The heat transfer initially reduces the temperature within the tunnel so that the single temperature sensing arrangement causes the temperature controller to provide increased energization of the heater elements of the infrared panels. This ultimately can produce an overshoot of the temperature within the oven so that food articles may be exposed to excessively high heat and, therefore, be burned upon the outside layers or interior portions of the articles may not be sufficiently cooked.
Thermal overshoot can be avoided by the use of a hearth of very high thermal mass, such as a very thick plate of steel. But such a hearth is undesirable because of the weight and expense. In the absence of such provision, the single temperature sensing arrangement currently employed, such temperature overshoot conditions may prevail or the temperature sensing arrangement will be responsive to only one end of the tunnel. For example, if no product is near the temperature sensing element, heating is reduced irrespective of the presence of a food article still within the tunnel. Consequently, the temperatures along the length of the tunnel may be essentially non-uniform and, at worst, subject to great extremes.
Another problem inherent in using a hearth having a high thermal mass is that there is a reduction in the speed of the oven to different masses in quantities of articles or different densities of articles passing through the oven. In other words, the oven will not be able to adjust quickly to provide the requisite conditions for the proper, uniform, and precise heating of the articles. Although it is desirable to provide low thermal mass and also to provide high watt density of the infrared heater panels in order to provide quick response, as the thermal mass decreases and the watt density increases, it has been found that the tendency of the oven to become thermally unstable increases drastically. Therefore, even tighter temperature regulation must be provided. Here, utilizing a single temperature sensing element of the prior art has not been found to be practical. Even if multiple temperature sensing elements are employed, impermissibly high temperature variations along the length of the tunnel may result and operation has not proven satisfactory for ovens of the character disclosed until the present invention.
Accordingly, an object of the invention is the provision of an improved tunnel oven for heating various products, and particularly food products, as the products are conveyed through a heating tunnel.
A further object of the invention is the provision of such a tunnel oven which employs an oven tier unit which is of a modular nature for providing the oven with a desired number of tiers, each having a tunnel-form oven chamber.
Another object of the invention is the provision of such a tunnel oven which provides preselected accurate temperatures in each tunnel of the oven for accurate, stable, predictable, repeatable heating of products therein.
Another object of the invention is the provision of such a tunnel oven which may optionally utilize either an in-line or return-type conveyor for conveying food products through the oven chamber.
A still further object of the invention is the provision of an improved tunnel oven providing high wattage infrared energy heating of products during their conveyance through a tunnel-form oven chamber yet which does not exhibit thermal instability, such as temperature overshoot, and particularly such a tunnel oven extremely well-suited to the precise, uniform, flexible, and high speed heating of food products of various types.
Yet another object of the invention is the provision of a multiple tier oven permitting substantial temperature differentials between adjacent tiers without detrimental effect upon conditions within the oven chamber of any one of tiers.
Among additional objects of the invention are the provision of a tunnel oven providing extreme flexibility in the preselection of heating conditions, including preselected temperatures, within an oven chamber tunnel; the provision of such a tunnel oven which is suited for uniform, precise heating of products under various product input conditions, various product input temperatures, and various rates of input; the provision of such a tunnel oven which is comprised of a simplicity of sturdy components conducing to longevity of usage and reliability; which may be produced economically; which permits economical field installation of additional oven tiers without difficulty or complexity; and which is of such simplicity that it does not require the services of a skilled individual for satisfactory operation.
Other objects will be in part apparent or are pointed our hereinbelow.